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Asymmetric catalysis of the Mukaiyama aldol reaction has been reported with complexes derived from Al, B, Sn (II), and (IV). The levels of asymmetric induction for the addition of propionate-, isobutyrate-, and acetate-derived silyl thioketene acetals to aldehydes parallel those obtained with chiral-auxiliary-based methodologies. However, silyl ketene acetals derived from O-alkyl acetates uniformly provide aldolates possessing lower levels of asymmetric induction. We have initiated a study aimed at the design and synthesis of chiral Ti (IV) complexes that catalyze the enantioselective Mukaiyama aldol of O-trimethylsilyl, O-methyl, and O-ethyl ketene acetals with aldehydes. We report herein a catalyst that consists of a tridentate ligand derived from 3, Ti (O'ⁱPr) ₄, and 3, 5-di-tert-butylsalicylic acid. This catalyst (2-5 mol %) furnishes aldol adducts in good yields and high levels of asymmetric induction (88-97% ee).
Carreira et al. (Thu,) studied this question.